2-Hydroxyarylimidazole-based colorimetric and ratiometric fluoride ion sensors

Abstract

Two novel 2-hydroxyarylimidazoles were developed as colorimetric and ratiometric sensors for the naked-eye detection of fluoride anions. The discriminating absorption and fluorescence responses to fluoride in acetonitrile as well as a methanol–acetonitrile solvent system were observed. The bathochromic optical changes are attributed to the deprotonated species evidenced from ¹H NMR titrations. Their selectivity for fluoride over acetate was successfully established in the methanol–acetonitrile mixture. A fluorene-bridged imidazole dimer, 4a demonstrated better response for fluoride than acetate in the neat acetonitrile. Moreover, the sensor 4a exhibited a larger ratio of fluorescence intensity response (I₄₈₉/I₄₃₁ = 51) than 4b (I₅₂₁/I₄₄₈ = 18) for fluoride in acetonitrile.